The present invention is directed to a composite material having high mechanical strength, increased glass transition temperature and high optical clarity or transparency, and to a process for producing the same. The composite material comprises a polymer with polar or polarizable groups, a layered silicate dispersed therein, and a substance such as a surfactant which controls the crystalline structure or molecular conformation of the polymer.
Glass and mineral reinforced polymers have been produced to increase the strength and barrier properties of many polymer applications. Recently, polymer silicate composites have been produced to attain the same degree of stiffness and strength with less ceramic content than comparable glass or mineral reinforced polymers. These materials take the form of polymer-ceramic nanocomposites with layered silicates as the inorganic phase. The materials design and synthesis rely on the ability of layered silicates to intercalate in the galleries between their layers a wide range of monomers and polymers.
Polymer layered silicate composites exhibit desirable properties for use in many commercial applications. These composites are stiffer and stronger and have better barrier properties than comparable glass or mineral reinforced polymers with less inorganic content, making them lighter in weight. Additionally, these silicate polymer composites have increased thermal stability and self-extinguishing properties. Composites with these characteristics have applications in paint, varnishes, lacquers, liquid resins, epoxies and the like.
Because the silicate-layered composites achieve composite properties at much lower volume fraction of reinforcement, they avoid many of the costly and cumbersome fabrication techniques common to conventional fiber or mineral reinforced polymers. Known methods of making silicate-polymer composites involve forming the polymer in situ or carrying out intercalation by melting. U.S. Pat. No. 4,455,382 to Wu teaches the production of a polar liquid to disintegrate a glass-ceramic body prior to reaction with organic polycations to form the interlayer of the crystals to form an organic-inorganic composite. U.S. Pat. No. 4,739,007 to Okada et al teaches the use of an aqueous swelling agent mixed with a crushed clay material followed by ion exchange with an organic cation previously mixed with the swelling agent to produce a composite material. U.S. Pat. No. 4,894,411 to Okada et al expands on this method by introducing a polyamide monomer as the organic cation followed by polymerization of the monomer by heating in the presence of a phenol or polyamine stabilizer. U.S. Pat. No. 4,810,734 to Kawasumi et al teaches mixing a layered clay with a swelling agent to form a complex that is mixed with a monomer to intercalate into the layered material followed by polymerization of the monomer to form a composite material. U.S. Pat. No. 5,955,535 to Vaia et al teaches the intercalation of a polymer into the galleries of layered silicate by heating the polymer-silicate mixture to above the melting or glass transition temperature of the polymer in the absence of a solvent. This last method produces the sought after polymer-clay composites with small visible flecks of the organically modified clay and other suspended impurity materials thereby limiting their potential use in applications requiring transparency and temperature resistance. Accordingly, a new method of producing polymer-clay composites resulting in a transparent composite with better thermal properties is desired.
The process of the present invention produces composite polymer clay materials in a process using a solvent to intercalate the polymer in the clay material without in situ formation of the polymer or melting of the polymer. This process provides less expensive synthetic pathways to produce composite polymer clay materials with enhanced strength and stiffness.
The process utilizes a solvent for fabricating modified polymer clay materials in a process that is cleaner than a melt fabrication process and therefore likely to result in a transparent final material. In this process, an aliphatic salt is introduced into the galleries of a layered silicate in the presence of a solvent. The polymer is then mixed with the silicate in the same or another appropriate solvent and the solvent is evaporated, resulting in an optically transparent modified clay with an intercalated polymer.